Photovoltaic devices can include semiconductor material deposited over a substrate such as glass, for example, with a first layer of the semiconductor material serving as a window layer and a second layer of the semiconductor material serving as an absorber layer. The semiconductor window layer forms a junction with the semiconductor absorber layer where incident light is converted to electricity.
Photovoltaic devices can also include a transparent conductive oxide (“TCO”) layer to conduct electrical charge. One TCO material which is often used is crystalline cadmium stannate. This is because of crystalline cadmium stannate's low sheet resistance and high light transmissivity.
One conventional method of forming crystalline TCO layer is to deposit an amorphous layer of cadmium and tin oxide onto a substrate and to then transform the deposited amorphous layer to a crystalline form. This is done by annealing the amorphous layer at a high temperature (e.g., typically a temperature greater than 550° C.), in a low oxygen partial pressure environment (i.e., an oxygen-deficient or reduced atmosphere) for a sufficient amount of annealing time (e.g., at least 10 minutes).
To provide the low oxygen partial pressure environment, current photovoltaic device manufacturing processes advocate forming the semiconductor window layer, which may be made of cadmium sulfide, on the amorphous TCO layer before it is annealed. Doing so deprives the amorphous layer of oxygen that may be available in an ambient processing atmosphere. In addition, the cadmium sulfide layer over the amorphous TCO layer encourages any oxygen that may be present in the amorphous TCO layer to diffuse out of it. Specifically, oxygen that diffuses out of the amorphous TCO layer may react with the cadmium sulfide to form cadmium oxide which may evaporate at temperatures of about 600° C. and above and sulfur dioxide which will diffuse into the deposition ambient. This reaction then produces oxygen vacancies in the amorphous layer. Each oxygen vacancy acts as an electron donor which, once the amorphous TCO layer is transformed to a crystalline form, helps with electrical conductivity. Thus, the window layer is used as a reducing agent because it creates the needed oxygen-deficient atmosphere that promotes the oxygen vacancies in the TCO layer.
However, forming the cadmium sulfide window layer on the TCO layer before the amorphous TCO layer is annealed requires a longer annealing time, or a higher annealing temperature or both than would have been needed otherwise to transform the amorphous layer to the crystalline form. Using high temperatures for long periods of time can damage glass substrates. For example, glass substrates will often begin to soften at a temperature of about 550° C. and above. Thus, subjecting the glass substrates to such a high annealing temperature (i.e., greater than 550° C.) for such a relatively long time (i.e., 10 minutes or more), increases the risk of damaging the substrates. Specifically, the glass substrates may begin to soften and warp at the high annealing temperatures applied for such long annealing periods of time. Further, the high annealing temperature has a tendency to ionize sodium atoms or molecules present in the glass substrates, which over time may diffuse to other layers of the devices. Diffusion of sodium ions in certain layers of the devices may adversely affect device performance. In addition, a long annealing time decreases productivity and also subjects the annealing chamber to conditions favorable to chamber degradation, which can require remediation. Finally, the high annealing temperature used to transform the amorphous layer into crystalline is one of many high temperatures to which the devices may be subjected while being processed. For example, other layers have to be annealed at high temperatures. Thus, the devices may be subjected to a plurality of high thermal cycles. These thermal cycles may weaken the glass and subject it to a high degree of breakage.
Accordingly, a method of transforming an amorphous TCO layer to a crystalline form which mitigates against these potential problems is desired.